A Faraday rotor used in an optical isolator, an optical circulator, or the like uses a Bi-substituted rare earth iron garnet single crystal (hereinafter, referred to as a “garnet single crystal” as occasion demands) that is grown according to a liquid phase epitaxy (LPE) method. The LPE method is a method of growing a single crystal, wherein a melt is prepared by putting a material in a crucible and melting the material and a garnet single crystal is epitaxial-grown by contacting a growing substrate with the melt. Since PbO is used in the melt along with B2O3, Bi2O3, or the like, Pb (lead) contained in the grown garnet single crystal is unavoidable.
Recently, in order to prevent an adverse effect of a highly harmful chemical material on the environment, there have been movements to restrict harmful material from being included in a product worldwide, and lead is also designated as a harmful material. An example of a restriction on harmful materials includes Restriction of Hazardous Substances (RoHS) enforced by the European Union (EU). Moreover, even a small amount of lead included in a garnet single crystal may cause environmental contamination, and this has been a problem.
Thus, in order to remove lead included in a garnet single crystal, it is required to grow a garnet single crystal from a melt that does not include lead. Specifically, in a garnet single crystal included in an optical isolator, a thickness for rotating a plane of polarization of light to about 45° was required for optical communication in a wavelength from about 1.3 μm to about 1.6 μm based on a relationship of a Faraday's rotation angle.
Also, along with such a predetermined thickness, it is important to reduce an insertion loss (hereinafter, referred to as “IL”), which is obtained by converting into the thickness generating the Faraday's rotation angle of 45°, to 0.10 dB or lower, which is a market demand for an optical isolator. When a garnet single crystal is grown from a conventional lead-containing melt, platinum (Pt) is used in a crucible for melting a material considering heat resistance and corrosion resistance. Here, when the garnet single crystal is grown in an atmosphere, platinum that is a crucible material is oxidized and dissolved in the melt, and a small amount of platinum is also included in the grown garnet single crystal as quadrivalent platinum ions (Pt4+).
Then, it is assumed that bivalent lead ions (Pb2+) and quadrivalent platinum ions (Pt4+) included in the garnet single crystal generate optical absorption, and bivalent or quadrivalent iron ions (Fe2+ or Fe4+) induced by the bivalent lead ions and quadrivalent platinum ions increase optical absorption in a wavelength band from 1.3 μm to 1.6 μm, thereby increasing IL.
Since the garnet single crystal is stable in trivalence, charge compensation is performed by using quadrivalent additives when bivalent impurities are excessive, and charge compensation is performed by using bivalent additives when quadrivalent impurities are excessive. It is thought that since the conventional garnet single crystal contains Pb, the Pb exists in the garnet single crystal as bivalent ions (Pb2+) and quadrivalent ions (Pb4+), and thus a contained balance of the bivalent ions (Pb2+) and the quadrivalent ions (Pt4+ and Pb4+) in the entire garnet single crystal is almost equally maintained. As a result, the generation of Fe2+ or Fe4+, which increases optical absorption, is low in the conventional garnet single crystal, and thus it is thought that IL is suppressed from being increased (assuming that Pb2+ content>Pb4+ content).
However, since a melt that does not include lead does not contain Pb2+ and Pb4+ at all, Pt4+ is relatively excessive in a garnet single crystal grown from the melt. Accordingly, Fe2+ ions, which are bivalent ions, are generated to perform charge compensation on the excessive Pt4+, and thus optical absorption is increased, thereby increasing IL.
Accordingly, a method of using gold (Au), instead of platinum, as a crucible material while growing a garnet single crystal from a melt that does not include lead has been suggested. For example, Patent Reference 1 suggests adding an alkali metal such as Na, instead of Pb, as a melt component that does not include lead.
Also, a technology of suppressing an increase in IL by adding Ca that is bivalent like Pb2+, instead of Pb2+, has been suggested (for example, refer to Patent Reference 2).